Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session C6: High Field Physics at Short Wavelengths |
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Chair: Barry Walker, University of Delaware Room: Minor Hall 125 |
Wednesday, May 20, 2009 2:00PM - 2:30PM |
C6.00001: Ultrafast processes and single shot imaging of clusters with intense soft x-ray radiation from the FLASH free electron laser Invited Speaker: For many potential experiments with free electron lasers it is of fundamental importance to study how the absorption and ionization properties of nanoscale systems develop in the short-wavelength strong-field domain. For such investigations, atomic clusters are ideal targets as hidden energy dissipation into surrounding media is virtually absent. We have performed first experiments about the soft x-ray laser pulse - cluster interaction with a combined spectroscopy and imaging approach. The new data show qualitatively different processes for (soft) x-ray pulses from the optical strong field regime. Electrons are emitted from the clusters in a direct multistep photoionization process and plasma type absorption is not significant. Resonant excitation of Xe clusters at 90 eV and power densities exceeding 10$^{14}$ W/cm$^2$ yield high charge states of up to 9$^+$. The investigation of core - shell gives evidence for efficient charge redistribution within the cluster, leading to explosion of the cluster outer layers and recombination of the nanoplasma core. For single-shot imaging of clusters with intense short wavelength radiation a new detector system has been developed. Mie calculations indicate that the optical constants of the clusters, which are inherently coupled to its electronic structure and thus charge states, change during the femtosecond pulse. The results show that ultra fast scattering is a promising approach to study transient states of matter on a femtosecond time scale. [Preview Abstract] |
Wednesday, May 20, 2009 2:30PM - 3:00PM |
C6.00002: Femtosecond Isomerization Dynamics in the Ethylene Molecule Invited Speaker: The ethylene molecule plays a fundamental and prototypical role for the understanding of photo-isomerizaton processes and particularly for ultrafast energy conversion through nonadiabatic transitions and state crossing via conical intersections. We have developed a high power femtosecond laser based pump-probe system to study femtosecond isomerization dynamics in various model molecules. By focusing 25-mJ laser pulses into a 5-cm-long xenon-filled gas cell, we can deliver about 10$^9$ photons per harmonic per pulse onto a target gas, with the photons ranging in energy from 8 to 40 eV. In this talk I will present the results of our studies of the dynamics in the excited ethylene cation (C$_2$H$_4^+$) using a high intensity high harmonic source. The dynamics in the excited ethylene cation leads, among other channels, to isomerization to the ethyledene configuration (CH$_3$CH$^+$), which is predicted to be a transient configuration for electronic relaxation. With an intense femtosecond EUV pulse as pump, and a NIR (near infra-red) pulse as probe, we measure a time scale of $45\pm10$ fs for formation of the transient ethylidene configuration (lifetime of $60\pm15$ fs ) through detection of the NIR-induced fragmentation to CH$_3^+$ and CH$^+$. Also, a H$_2$-stretch transient configuration (believed to succeed ethylidene), yielding H$_2^+$, is found to be populated after $100\pm10$ fs. These studies were also extended to excited state dynamics in the neutral ethylene using a recently developed split mirror technique enabling XUV pump - XUV probe capability. In order to achieve this we optimized our high harmonic system for high power in order to produce a very intense source of high harmonics that allows multiphoton (XUV) absorption by a single molecule. In particular, we were able to measure two-photon double-ionization of Ethelyne and argon and three-photon double ionization of neon. [Preview Abstract] |
Wednesday, May 20, 2009 3:00PM - 3:30PM |
C6.00003: Ultrafast electron dynamics in finite atomic systems Invited Speaker: With the LCLS free-electron-laser source at Stanford coming on line, new possibilities of light-matter couplings arise. This talk will focus on ultrafast electron dynamics in clusters as a result of a sudden removal of inner shell electrons by photo ionization through a high frequency (about 10 keV) XFEL pulse. A detailed understanding of this electron dynamics is crucial to realize one of the prime goals envisaged with XFEL beams: single molecule imaging. We will show that field induced electron migration influences the motion of the nuclei and is therefore important for imaging. The electric field under discussion is generated by the (photo)-ionized atoms. Time permitting another -- quite surprising -- ultrafast multi-electron process will be introduced. It can be triggered by a few seed atoms implanted in a rare gas cluster when illuminated by a standard 800nm strong laser pulse with 50 fs duration. Preferential ionization of the seed atoms (with ionization potential lower than those of the cluster) again creates a strong electric field gradient which removes very quickly (on a time scale of 1 fs) many electrons bound to atoms of the cluster. We will explain the effect which occurs for both, linear and elliptic polarization of the laser. [Preview Abstract] |
Wednesday, May 20, 2009 3:30PM - 4:00PM |
C6.00004: Ultrashort non-linear interactions of x-rays with atoms Invited Speaker: Upcoming x-ray free electron laser sources (XFELs) will open the pathway to study non-linear optical effects in the x-ray regime for the first time. In this contribution we will present model calculations of resonant and non-resonant interaction of XFELs with atoms, tailored to predict and plan first experiments on atomic neon at the Linac Coherent Light Source at Stanford. The most fundamental textbook example of a resonant, non-linear interaction of photons with atoms is Rabi flopping -- the coherent, oscillatory population transfer in a quasi two-level system by absorption and stimulated emission of photons. In the x-ray regime, resonant excitation of an inner-shell electron results in a core-excited state, which decays on the time-scale of a few fs. Ultrahigh intensities are therefore needed, to efficiently drive the back and forth transitions, which will become available with upcoming XFEL sources. The relatively long durations of these sources, in the range of 100 fs, will, however, hamper to study this fundamental effect in the time-domain. Shorter x-ray pulses of fs or sub-fs duration are therefore needed to study quantum optical processes involving core electrons. A possible route to sub-fs pulses with intensities comparable to XFEL sources is to use XFELs to pump an inner-shell atomic x-ray laser. Thereby a variety of different lasing pulse structures can be realized: isolated, transform limited x-ray bursts of sub-fs duration, pulses of 10 fs duration with increased temporal coherence as compared to XFELs and trains of fs pulses of different wavelength. The latter would open the pathway for ultrashort two color pump-probe sources in the x-ray regime. Self-consistent gain and amplification calculations and possible applications of this new kind of multi-color x-ray source will be discussed. [Preview Abstract] |
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